The UK’s information commissioner, Elizabeth Denham, said: “The vast number of people affected by this cyber attack is staggering and demonstrates just how severe the consequences of a security hack can be.

“The US authorities will be looking to track down the hackers, but it is our job to ask serious questions of Yahoo on behalf of British citizens and I am doing that today.

“We don’t yet know all the details of how this hack happened, but there is a sobering and important message here for companies that acquire and handle personal data.

“People’s personal information must be securely protected under lock and key – and that key must be impossible for hackers to find.”